SUMMARY The loss of tissue homeostasis and regenerative capacity with age underlies some of the most challenging health issues in the elderly. A major contributor to age-related decline in the structure and function of many tissues is the loss of stem cell function that occurs during the aging process. To be able slow, arrest, or even reverse those age-related declines in stem cell function holds would represent a major therapeutic advance in the growing field of regenerative medicine. The primary focus of this Program is to understand the molecular basis of age-related changes in stem cell function with the underlying premise that such an understanding is likely to reveal ways to target those changes in order to enhance aged stem cell function. Based largely on the work from the laboratories of this Program, there is increasing evidence of profound epigenetic changes that occur in stem cells as they age and that underlie many functional changes. A central conceptual theme that is woven throughout the proposal is the notion that these changes are not only mediators of cellular function but are amenable to being restored to a more youthful state through the process we call ?epigenetic rejuvenation?. In addition, we explore how the epigenetic changes confer upon stem cells the characteristics that may be responsible for selection of subsets of cells across the lifespan by an ever changing adaptive landscape and result in a population of distinct stem cells in the aged milieu. To examine those concepts experimentally, this Program includes three Projects that focus on somatic stem cells from three different tissues that differ in terms of their homeostatic turnover and regenerative capacity ? blood (high turnover, highly regenerative), muscle (low turnover, highly regenerative), and brain (low homeostatic, minimally regenerative). The Projects are integrated in their shared approaches to understanding the epigenetics of stem cell aging using cutting-edge technologies, including single cell transcriptome and epigenome analysis, modified CRISPR/cas9 technology for locus-specific epigenetic modifications, and in vivo lineage studies to explore the dynamics of stem cell populations during aging. These Projects are supported by three essential Cores ? an Administrative Core to facilitate the interactions among the laboratories, a ?Rejuvenation Strategies? Core for our shared animal studies to explore epigenetic rejuvenation, and a Bioinformatics Core to serve as the essential hub for storing, processing, visualizing, analyzing, and sharing data from the three Projects. Overall, the investigators who are Project Leaders and Core Directors are internationally recognized experts in these areas and bring to the Program the full breadth of expertise, creativity, and records of accomplishment to assure an integrated, innovative, and highly successful Program to explore the molecular mechanisms of stem cell aging.